The White House’s computer network was breached by hackers sponsored by the Russian government, administration officials told the Washington Post. The cyberattack gave those responsible access to unclassified information and caused temporary network disruptions, though it has not resulted in the loss of any national security secrets.

American officials were first alerted to the attack two to three weeks ago by an unnamed ally. The FBI, Secret Service and National Security Agency are investigating details of the infiltration, which comes only weeks after Director of National Intelligence James Clapper said at a University of Texas forum that he worries “a lot more about the Russians” than Chinese hackers.

“In the course of assessing recent threats, we identified activity of concern on the unclassified Executive Office of the President network,” a White House official told the Post, speaking on the condition of anonymity due to an ongoing investigation. “We took immediate measures to evaluate and mitigate the activity.... Unfortunately, some of that resulted in the disruption of regular services to users. But people were on it and are dealing with it.”

White House staff has experienced intermittent intranet and virtual private network outages as part of preventative measures. Yet defensive outages rarely last more than a few hours before they’re no longer necessary. Officials said the length of the downtime has been an indication of the attack’s strength.

“On a regular basis, there are bad actors out there who are attempting to achieve intrusions into our system,” a second official told the Post. “This is a constant battle for the government and our sensitive government computer systems, so it’s always a concern for us that individuals are trying to compromise systems and get access to our networks.”

Hackers from Russia and Eastern Europe have become the bane of administration security officials in recent months. They’ve been blamed for an intrusion at JPMorgan Chase & Co. that resulted in the largest known data loss ever, as well as recent attacks on the Nasdaq and the purported theft of 5 million Gmail usernames and passwords.